docs: def Smith(x, rhol, rhog):
      r'''Calculates void fraction in two-phase flow according to the model of 
      [1]_, also given in [2]_ and [3]_.
  
      .. math::
          \alpha = \left\{1 + \left(\frac{1-x}{x}\right)
          \left(\frac{\rho_g}{\rho_l}\right)\left[K+(1-K)
          \sqrt{\frac{\frac{\rho_l}{\rho_g} + K\left(\frac{1-x}{x}\right)}
          {1 + K\left(\frac{1-x}{x}\right)}}\right] \right\}^{-1}
  
      Parameters
      ----------
      x : float
          Quality at the specific tube interval []
      rhol : float
          Density of the liquid [kg/m^3]
      rhog : float
          Density of the gas [kg/m^3]
  
      Returns
      -------
      alpha : float
          Void fraction (area of gas / total area of channel), [-]
  
      Notes
      -----
      [1]_ is an easy to read paper and has been reviewed.
      The form of the expression here is rearranged somewhat differently
      than in [1]_ but has been verified to be numerically equivalent. The form
      of this in [3]_ is missing a square root on a bracketed term; this appears
      in multiple papers by the authors.
  
      Examples
      --------
      >>> Smith(.4, 800, 2.5)
      0.959981235534199
  
      References
      ----------
      .. [1] Smith, S. L. "Void Fractions in Two-Phase Flow: A Correlation Based 
         upon an Equal Velocity Head Model." Proceedings of the Institution of 
         Mechanical Engineers 184, no. 1 (June 1, 1969): 647-64. 
         doi:10.1243/PIME_PROC_1969_184_051_02.  
      .. [2] Xu, Yu, and Xiande Fang. "Correlations of Void Fraction for Two-
         Phase Refrigerant Flow in Pipes." Applied Thermal Engineering 64, no. 
         1-2 (March 2014): 242–51. doi:10.1016/j.applthermaleng.2013.12.032. 
      .. [3] Dalkilic, A. S., S. Laohalertdecha, and S. Wongwises. "Effect of 
         Void Fraction Models on the Two-Phase Friction Factor of R134a during 
         Condensation in Vertical Downward Flow in a Smooth Tube." International 
         Communications in Heat and Mass Transfer 35, no. 8 (October 2008): 
         921-27. doi:10.1016/j.icheatmasstransfer.2008.04.001.
      '''
      K = 0.4
      x_ratio = (1-x)/x
      root = ((rhol/rhog + K*x_ratio) / (1 + K*x_ratio))**0.5
      alpha = (1 + (x_ratio) * (rhog/rhol) * (K + (1-K)*root))**-1
      return alpha